Modern gas turbine engines include control systems, for example, a full authority digital electronic control, which include portions for measuring, calculating, and predicting temperature of fluid flow therein, such as air temperatures, for example. A conventional temperature sensor is an electromechanical device which typically includes a conventional thermocouple or conventional resistive thermal device (RTD) for measuring temperature and generating an electrical signal proportional thereto. The thermocouple or RTD is typically embodied in a structural member for protecting it from the fluid environment. In order to sense the temperature of the fluid, heat from the fluid must first flow through the structural member and then to the thermocouple or RTD. During transient operation, where the temperature of the fluid is either increasing or decreasing in value, the RTD and structural member necessarily introduce a thermal lag in response until the member and the fluid reach a steady state equilibrium
The sensor structure, as well as any additional structure surrounding the sensor, such as a protective sheath or strut in which the sensor is supported, for example, also introduces what is conventionally known as heat soak errors in the measured temperature signal. More specifically, during transient operation wherein the fluid is either increasing or decreasing in temperature, such structures thermally lag in response thereto and are initially either colder than or hotter than the fluid and thereby introduce a lag by providing heat into or away from the sensor during such transient operation.
One prior art method for compensating for thermal lag in the temperature sensor is to introduce a predetermined lead into the measured temperature signal. However, such lead also introduces a substantial amount of noise which requires the use of a filter to reduce or cancel the noise. And, such filter also slows the response, or tracking, of the temperature sensor.
Another prior art arrangement for compensating for transient errors in measured temperature includes open loop parallel compensation for prediction of temperature as described in more detail hereinbelow. In open loop parallel compensation, a separate loop in the control system is provided parallel to the path providing a measured temperature signal from the temperature sensor. The parallel loop includes predetermined schedules proportional to rotor speed and another engine temperature signal for mathematically simulating or predicting the temperature of the fluid at the temperature sensor. The open loop calculates the lag introduced by the temperature sensor using a sensor model, and a heat soak model of engine structures adjacent to the sensor and adds the value of the lag therefrom to the measured temperature to obtain a corrected, or predicted, temperature having reduced transient error.
The performance of the open loop compensation method is dependent, in part, upon the accuracy of the heat soak and sensor models. For given engine and temperature sensor statistical populations, the calculations in the models reflect only nominal, or average, performance which is embodied in the open loop method. Since in actual practice individual temperature sensors and adjacent structures vary from those represented by the nominal engine and sensor models, the temperature prediction can still vary significantly from the true temperature to be measured.
In one open loop parallel compensation method, the temperature from the temperature sensor is predicted based upon rotor speed. In an exemplary situation, such as where an aircraft flies in a rainstorm, the temperature of the air will be reduced, but the rotor speed will remain essentially constant. Where the engine model includes a predetermined schedule providing a prediction of temperature proportional to rotor speed, that schedule does not include such a situation affected by rain, and therefore the ability of the open loop parallel compensaton to predict the actual temperature is necessarily reduced, thus not being able to fully compensate for the error in the measured temperature.
Furthermore, the conventional open loop parallel compensation method is independent of the temperature measuring path and thus is unable to anticipate the rate of change of the measured temperature during transient operation. Accordingly, the ability of the open loop parallel compensation method to predict the actual engine temperature is limited.
Although the open loop parallel compensation method provides satisfactory temperature prediction in general, in certain applications, it is desirable to have improved temperature compensation for more closely matching the predicted temperature with the actual temperature experienced in the engine.